Doping and milling of granular silicon

ABSTRACT

Doped silicon particles, including powder suitable for plasma spraying semiconductor devices, is formed by liquid doping applied to larger particles, which are then milled to a smaller size. Doped or undoped silicon may be milled by a roller mill including silicon rollers and advantageously having feed and collection systems formed of silicon and operated in a nitrogen ambient. A two-stage system includes sieving the rolled product for further size reduction in a jet mill.

RELATED APPLICATION

This application claims benefit of provisional application 61/165,218,filed Mar. 31, 2009.

FIELD OF THE INVENTION

The invention relates generally to producing silicon grains forming apowder useful for plasma spraying semiconducting devices such as solarcells. In particular, the invention relates to both the doping andmilling of such silicon powder.

BACKGROUND ART

Integrated circuits based upon semiconducting silicon haveconventionally been formed in monocrystalline silicon wafers cut fromingots grown by the Czochralski method, which includes pulling the ingotfrom a melt of pure silicon. Solar cells can also be made in suchwafers, but the conventional monocrystalline silicon wafers aregenerally considered to be too expensive for solar cells to beeffectively and widely deployed as an economical replacement forcommercial power. As a result, much recent effort has been directed todeveloping economical techniques for depositing a semiconductor siliconlayer on another substrate.

One such technique involves plasma spraying thin layers of silicon ontoforeign substrates, as described by Zehavi et al. in U.S. patentapplication Ser. No. 12/074,651, filed 5 Mar. 2008 and now published asU.S. patent application publication 2008/0220558. Zehavi discloses animproved design of the gun nozzle in U.S. patent application Ser. No.12/720,123, filed 9 Mar. 2010. This technique includes injecting siliconpowder into the flame of a plasma spray gun and directing the flame andentrained silicon toward the substrate. The silicon powder is melted andperhaps vaporizes in the flame but quickly solidifies when it strikesthe substrate and forms a silicon layer it.

However, to form a photovoltaic solar cell or any type of significantsemiconducting device, the device must includes layers of silicon ofdifferent conductivity types.

Further, the size of the powder used in plasma spraying must becontrolled within a fairly narrow range of small dimension to facilitateprocessing. It have been found desirable to mill or otherwise reduce thesize of silicon particles otherwise available on the market. However,the milling must maintain the purity of the silicon. Zehavi et al. havedisclosed a jet mill for reducing the size of the silicon particles inUS patent application Ser. No. 11/782,201, filed 24 Jul. 2007, nowpublished as U.S. patent application publication 2008/0054116, andincorporated herein by reference. In order to maintain the purity of themilled silicon, the jet mill has walls composed of silicon so that themilling process does not incorporate non-silicon wall material into themilled silicon.

However, further development work in our laboratory has suggested thetypical commercially available silicon particles, such as BB pellets,used as feedstock to the mill rapidly degrade the silicon parts of themilling chamber. Although the parts can be easily replaced with newsilicon parts, silicon parts are generally expensive. If the final useof the milled powder is forming solar cells, it is important that allstages of the manufacturing process be economical to allow solar cellsto compete with other more conventional forms of electrical power.

SUMMARY OF THE INVENTION

According to one aspect of the invention, silicon particles are doped ton-type or p-type semiconductivity by exposure to a liquid doping agentand then ground to smaller size. The smaller-sized particles produced bymechanical milling may be further reduced in size by jet milling to apowder of size suitable for plasma spraying of silicon, for example, ofdiameter of 1 to 5 microns.

The doping concentration in the final silicon can be controlled bymilling undoped silicon particles to the same size as the doped groundsilicon particles and mixing the two batches of particles inpredetermined proportion to achieve the desired doping concentration.

According to another aspect of the invention, silicon particles arecrushed to smaller size by a roller mill including two counter-rotatingsilicon rollers.

The roller milling system may include a feed system which reciprocatesalong the lengths of the rollers in delivering silicon particles to therollers. The feed system may include a linear funnel having a linear,generally rectangular outlet positioned away from the gap between therollers. The funnel, a vibrating feeding trough for the funnel, and acollector pan positioned beneath the inter-roller gap are advantageouslycomposed of silicon.

The roller milling system and its feed system are advantageouslydisposed within an environmental chamber back filled with an inert gassuch as nitrogen. If the crushed particles are then jet milled to a yetsmaller size, the jet mill is advantageously placed in the sameenvironmental chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a spraying apparatus forliquid doping of silicon particles according to one aspect of theinvention. FIG. 2 is an orthographic view of one embodiment of a rollermill of another aspect of the invention.

FIG. 3 is an exploded orthographic view of the roller mill of FIG. 2.

FIG. 4 is a yet further exploded view of FIG. 3.

FIG. 5 is a cross-sectional view of the chamfered end of one of therollers of FIG. 1.

FIG. 6 is a schematic cross-sectional view of a feeding system for theroller mill of FIG. 2.

FIG. 7 is a schematic representation of a environmental chamber in whichthe roller mill of FIG. 2 is disposed in an inert ambient.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Different aspects of the invention include mechanical grinding orcrushing of silicon pieces to small particles of high purity and ofcontrolled size and the semiconductor doping of such silicon particles.The mechanical grinding may be followed by an optional sieving andsubsequent jet milling of the particles into yet smaller silicon powder.However, the silicon feedstock must be highly pure and the purity shouldbe maintained during the grinding process.

The semiconductor industry has promoted the development of economicalproduction of electronic grade silicon (EGS) of very high purity. In theusual Siemens process, gaseous precursors of hydrogen andtrichlorosilane are injected into a reactor containing a hot seed rod ofsilicon. The precursors in a chemical vapor deposition (CVD) processdeposit onto the seed rod as growing layers of polysilicon silicon toform a rod or ingot of EGS, also called virgin polysilicon. The growthconditions favor the formation of high stress in the ingots. At the endof growth, the ingots are cooled and hit by a sharp instrument on theorder of a hammer so that the ingot shatters into irregular chips orshards of silicon of size on the order of 2 mm to 20 mm typically havingirregularly arranged sharp corners. The silicon chips are used to asfeedstock to fill the crucible, which is then heated to melt the siliconfrom which monocrystalline silicon ingots are pulled in the Czochralskiprocess. A related fluidized bed process uses approximately the samechemistry to produce a continuous output of generally spherical pelletsor BBs of diameter on the order of 1 to 3 mm.

Either the chips or the pellets should be milled to the smaller sizesrequired for plasma spraying or otherwise applying a layer of siliconpowder to a substrate. However, it is advantageous to dope the siliconfeedstock before the feedstock is reduced in size to the powder. Solarcells and other semiconductor devices require doping levels in thevicinity of 10¹⁴ to 10¹⁹ cm⁻³, which compares to a silicon concentrationof 10²² cm⁻³ so that only a very small fraction of dopant incorporationis desired and that needs to be fairly closely controlled.

In one aspect of the invention, the silicon shards or pellets are dopedby spraying them with a liquid doping agent. An exemplary embodiment ofa spraying system schematically illustrated in the cross-sectional viewof FIG. 1 includes a showerhead 2 supplied with a liquid doping agentfrom a tank 4 and positioned over a container 6 having an open top andcontaining a thin layer of silicon particles 8 to be doped. The liquiddoping agent exits the apertures of the showerhead 2 as jets andpreferably as a fine mist evenly applied to the silicon particles 8. Thespray misting effectively coats the silicon particles with a thin filmof the liquid dopant with relatively little pooling of the liquid underthe particles. Unillustrated metering valves disposed between the tank 4and the showerhead 2 and an associated controller determines the totalamount of liquid doping agent applied to a known quantity of siliconparticles 8.

An example of a p-type liquid doping agent is boric acid. Boric acid isitself a solid at room temperature, typically in powder form, having aformula B(OH)₃, but is readily soluble in water or some alcohols. Thisis the ortho form of boric acid. The tetra form H₂B₄O₇ is also calledboric acid. The ortho boric acid is dissolved in a solvent to a knownconcentration, e.g. 0.1365 mol/l in deionized water. A known amount ofthe solvent is then sprayed onto a known amount of the silicon chips orpellets in an amount that does not significantly drain from the siliconparticles. The solvent is then evaporated. Heating of the boric acidcoating tends to convert it to boron oxide (B₂O₃), which has asublimation temperature of 3000° C. However, it reacts with siliconaccording to

2B₂O₃+3Si→3SiO₂+4B.

The boron readily diffuses in silicon at higher temperatures, especiallythose required for recrystallization, and thus can assume a dopinginterstitial position in the silicon lattice. Although the reaction anddopant diffusion may be performed by an anneal of the coated siliconparticles, when the final powder is used for plasma spraying, the plasmaspraying itself drives the reaction and the diffusion can easily occurin the liquid phase Also, the zone melt recrystallization (ZMR)contemplated for producing solar cells will also produce the requiredhigh temperatures for reaction and diffusion. The boron incorporationrate depends on the processing and may be in the vicinity of 10 to 20%.Other boron sources include boron tribromide (BBr₃) and trimethylborate((CH₃O)₃B but other sources and other p-type dopants in liquid form arepossible.

An example of a corresponding n-type liquid doping agent is phosphorusoxychloride (POCl₃), which has a melting point of 2° C. and a boilingpoint of 105° C. and may be used by itself or in solution with water oralcohol. The POCl₃ is converted to P, which diffuses into the silicon.Other liquid sources of phosphorous and other n-type dopants in liquidform are possible.

The silicon particles may be immersed in liquid dopant or the solutionincluding the dissolved doping agent, but the amount of dopant adheringto the particles is more difficult to control during immersion.

Very low doping concentrations can be better controlled by doping only aknown fraction of the silicon particles. The preferred method dopes somesilicon particles in one batch but leave other silicon particles undopedin another batch. Both batches are respectively milled or otherwisepulverized to produce similarly sized but segregated powders of muchsmaller size. The doped and undoped powders are then mixed in knownproportions and homogenized as much as possible. The average dopingconcentration of mixed powder is thereby reduced according to thefractions of doped and undoped powders.

The liquid doping procedure is difficult to control for a precisepredetermined doping level. Controlled doping can be achieved bypreparing a mixed or unmixed batch of doped powder as a doping runbatch, plasma spraying the doping run batch, and measuring theresistivity of the resulting sprayed silicon film, which is easilyconverted to doping concentration. The doping concentration orresistivity of the doping run sample is then compared to the desiredconcentration or resistivity, and a production run batch of siliconpowder is prepared from fractions of doped powder of the doping runbatch and undoped powder in amounts determined by the comparison.

The shards or pellets can be milled in at least two differentprocedures. In the first, relatively small silicon particles, such as BBpellets, are fed into a jet mill in which the particles are entrained ina vortex and strike each other or the walls of the milling chamber toprogressively reduce the size of the particles as was described abovefor application publication 2008/0054106. However, most commonlyavailable silicon particle feedstock has larger size than desired andprematurely degrades the expensive silicon parts of the milling chamber.

It is desirable to first reduce the size of silicon feedstock in aroller mill. The feedstock may be either the millimeter sized CVDpellets or the larger and more irregularly shaped shards from fracturedboules of virgin polysilicon (electronic grade silicon).

The roller milling process of one embodiment of the invention crushesthe particles between two closely spaced counter-rotating rollers havingsurfaces composed of high-purity silicon. In one embodiment illustratedin the orthographic view of FIG. 2, a roller mill 10 includes tworollers 12, 14, each right cylindrically shaped about respective rolleraxes and composed of high-purity silicon, such as virgin polysilicon(electronic grade silicon) as described by Boyle et al. in U.S. Pat. No.6,617,225 or float zoned silicon. Although silicon surfaces on a rollerbody of another material suffice, it has been found that solid elementalsilicon can be easily machined into rollers which withstand the rigorsof roller milling. Elemental silicon has a silicon content of at least95 at % and has a majority of the silicon atoms bonded to one anotherthrough tetrahedral covalent bonds. The right cylindrical surfaces maybe smooth or one or both of the rollers 12, 14 may have patternedsurfaces to facilitate grinding by more effectively gripping theparticles. The rollers should be machined to very tight tolerances toallow a very small minimum gap between them of the order of 25 microns.To achieve such a gap and allow the rollers to rotate without binding,the circularity should be 1/1000 inch (25 microns) or less, that is,outer diameters that differ ±0.0005 inch from the average. Thecircularity is preferably substantially less than the desired size ofground powder since the circularity limits how closely the rollers canbe separated. Shafts are fixed on the rollers 12, 14 and portionsextending from the opposed roller ends are mounted on a metal frame 16through bearings supported in respective bearing housings or carriers18, 20 (only two of the four being illustrated). The carriers 18, 20 arehorizontally movable over short distances within windows 22 in the frame16 to adjust the gap between the rollers 12, 14 and their parallelismbut the carriers 18, 20 can be fixed at their desired horizontalpositions. The rollers 12, 14 are mounted within a central opening ofthe frame 16 with a generally horizontal orientation with the axes ofthe rollers 12, 14 generally parallel in a same plane, which is eitherhorizontal or a few degrees away from horizontal. Two motors 24 driverespective reducing gears 26, which are coupled through respectiveflexible shaft couplings 28 to the shafts of the respective rollers 12,14 and rotate them in opposite directions, that is, counter-rotatingrollers, at approximately the same rotation rate of about 2 to 5 rpm forrollers of diameter of about 4 to 6 inches (10 to 15 cm). However, ithas been found that slightly different rotation rates between therollers 12, 14 operates better and more smoothly and avoids the pelletsbinding the rollers 12, 14. Clutches or torque limiters may beinterposed between the motors 22, 24 and the rollers 12, 14. Moresimply, the current to the motors 24 may be controllably limited. In animproved design, motor torque may be specified and adjustedperiodically.

The flexible shaft couplings 28 are intended to allow some angularmisalignment or nutation between the shafts of the rollers 12, 14 andthe associated motors 24 as the hard silicon particles forced betweenthe rollers 12, 14 exerts great force on the rollers 12, 14 and causesthem to separate. The flexible shaft couplings 28 may be a two-stagebellows coupling with a rigid ring attached between them. The two setsof bellows provide rigid rotational torque but allow the rotational axesto bend. Thereby, the motor and roller shafts maybe slightly inclined orradially displaced from each other. Such flexible shaft couples areavailable from Nuland Manufacturing Co. of Marlborough Mass.

As illustrated in more detail in the enlarged orthographic view of FIG.3, end plates or shields 30 are mounted to the inside of the frame 16through compression springs 32 connected between the carriers 18, 20 andthe shields 30 which bias the shields 30 against the axial ends of thetwo rollers 12, 14. However, the shields 30, which may be generallyflat, include apertures for the shafts of the rollers 12, 14. Thesprings 32 cause the shields 30 to abut the ends of the rollers 12, 14,to prevent unground and partially ground particles from falling off theends of the rollers 12, 14 and thereby shield the bearings from thegrinding dust. The shields 30 may also be composed of high-puritysilicon, such as virgin polysilicon, so that any grinding between therollers 12, 14 and the shields 30 produces only high-purity siliconparticles and thus protects the rollers 12, 14 from metalliccontamination. However, the secondary grinding should be minimized andthere may be a more complexly shaped interface between the rollers 12,14 and the shields 30.

It is advantageous to form chamfers 34 in the corners of the rollers 12,14 adjacent the shields 30, as illustrated in the cross-sectional viewof FIG. 5. The chamfers 34 reduce breakage of the rollers 12, 14contacting the shields 30 as they counter-rotate about roller axes 12 a,14 a.

Because silicon is prone to sensitive to shock and to cracking andfracturing, soft plastic such as Teflon may be interposed betweensilicon and metal parts.

Each of the four carriers 18, 20 supporting the rollers 12, 14 and theirmotors 24 is horizontally guided along four horizontal slots 42,illustrated in detail in the orthographic view of FIG. 4, in thecarriers 18, 20 closely passing the thread bodies of socket head capscrews 44 screwed into the frame 16. In view of the large lateral forcesproduced by the crushing, the holding power of the cap screws 44 can beincreased by roughening one or both engaging surfaces at the interfacebetween the carriers 18, 20 and the frame 16. Other guiding means suchas square keys engaged in horizontally extending keyways. The carriers18, 20 are horizontally movable through adjustment mechanisms. Theadjustment mechanisms for the carriers 20 associated with the secondroller 14 may be two simple knob 46 and attached threaded rod screwedinto the respective carrier 20 and axially retained in the frame 16.Once the desired orientation of the first roller 14 has beenaccomplished, cap screws 44 on the carriers 20 are tightened to fix thefirst roller 14 in that orientation. The adjustment mechanism for thecarriers 18 associated with the first roller 12 may be more complex.Threaded rods 50, illustrated more clearly in the yet further enlargedorthographic view of FIG. 4, are threaded into the respective carriers18. Their other ends are axially retained in the frame 16 and connectedto wheel gears 52, 54, each engaged through respective worm gears 56, 58on a rotary shaft 60 having an adjustment handle 62 on its end tosimultaneously and equally move both carriers 18 associated with thefirst roller 12 in the horizontal direction toward or away from thefirst roller 14 to thereby provide a tandem adjustment mechanism.

In one optional mode of operation, the wheel gears 52, 54 are coupledtogether and the carriers 18 associated with the first roller 12 arelocked in place by their cap screws 44. The two carriers 20 associatedwith the second roller 14 are unlocked by loosening their cap screws 44,and the two handles 46 adjust the position of the unlocked carriers 20until the two rollers 12, 14 closely engage along their entire lengths.As a result, the axes of the first and second rollers 12, 14 areparallel and the gap between them is essentially zero. The cap screws 44on the carriers 20 associated with the second roller 14 are thentightened to lock the second roller 14 into its final position.Thereafter, any rotation of the adjustment handle 62, assuming the capscrews 48 on the carriers 18 of the first roller 12 are loosened, causesthe gap between the two rollers 12, 14 to change but to be uniform alongthe lengths of the rollers 12, 14. In practice, it has been foundsufficient to use a feeler gauge between the rollers 12, 14 to establishthe gap, for example, at 100 microns and to thereafter lock the carriers18, 20 and associated rollers 12, 14 in place with or without the use ofthe tandem adjustment mechanism. It is understood that the cap screwsand adjustment handle can be replaced by electro-mechanical means tomake the desired adjustments and to fix them in place. The locking ofthe carriers 18, 20 can be further improved by placing lock nuts on thetwo threaded rods 50 associated with the carriers 18 of the first roller12 and on two unillustrated rods associated with the carriers 20 of thesecond roller 14. All the threaded rods are axially retained in theframe 16 but threaded into the respective carriers 18, 20 to formrespective worm drives. Once the desired inter-roller gap isestablished, the lock nuts are tightened against the associated carriers18, 20.

Once the gap is selected and fixed, preferably by tightening the capscrews 44 and lock nuts, silicon particles are loaded into the V-shapedregion between the tops of the rollers 12, 14, and the counter-rotatingrollers 12, 14 crush the particles into finer sized particles, whicheventually fall through the gap between the rollers 12, 14 and arecollected in an unillustrated pan positioned beneath the gap between therollers 12, 14.

For purposes of this invention, grinding and milling are equivalentterms unless specified otherwise and crushing with a roller mill is aspecial case of milling.

The size of the inter-roller gap may be varied but fine powder isproduced for differently sized BBs for a variety of gap sizes. Gaps assmall as 100 microns have been successfully tested and with improvedroller circularity can be reduced further. The powder loading needs tobe carefully controlled. If too few particles are loaded, the conversionrate or yield decreases. If too many particles are loaded, the rollersjam. It is thus desirable to constantly feed particles to the rollers,for example, by a conveyor or elevator. However, the particles should beevenly distributed along the length of the rollers.

An embodiment of a feedstock supply system 70, illustrated in thecross-sectional view of FIG. 5, is generally positioned above the tworollers 12, 14 separated by a gap 72 and counter-rotating about theirrespective centers to force silicon feedstock fed from above downwardlythrough the narrow gap 72, thereby milling and crushing the feedstock tosmaller sized particles. A linear funnel 74 formed of two inclinedsidewalls extending into the plane of the illustration parallel to theaxes 12 a, 14 a of the rollers 12, 14 and having closed ends. The linearfunnel 74 includes at its bottom a outlet slot 66 extending linearlyparallel to the axes of the rollers 12, 14. A shield 78 extends on thesides away from the outlet 76 to confine any powder to the area abovethe inter-roller gap 72. The funnel outlet slot 76 is preferablypositioned vertically above one of the rollers 12, 14 away from theinter-roller gap 72 so that any particles broken in the crushing or evenunground BB pellets do not fly upwardly through the funnel outlet slot66. The funnel 74 and shield 78 are advantageously formed of high-puritysilicon such as virgin polysilicon. Thereby, upwardly flying particlesstriking the funnel 78 or shield 78 do not ablate contaminants fromthem.

An inclined V-shaped trough 80 is positioned with its open endvertically above the funnel 74 and its closed end 84 supported on avibrator 86. Unillustrated feeder means continuously or intermittentlysupply silicon particles to the closed end 84 of the trough 80, which isinclined upwardly toward the open end 82. This portion of the feedstocksupply system 70 has been described in aforecited patent publication2008/0054116, which should be consulted for further detail. As describedthere, the vibration causes small particles to march up the bottom ofthe inclined trough 80 and fall from its open end 82 as fed particles88, which pass through the funnel outlet 76 to be crushed to smallersize by the rotating rollers 12, 14. Crushed particles 90 fall into acollector pan 92 and develop into a mound 94 of ground silicon powder.Preferably the trough 80 and collector 90 are also made of high-puritysilicon although other high-purity material such as polypropylene orTeflon may be used for the collector pan 80.

The vibrator 86 is mounted on an axial stage 94 which reciprocates inthe axial direction of the two rollers 12, 14 such that siliconfeedstock is distributed along nearly the entire lengths of the rollers12, 14.

Silicon is highly prone to oxidation, especially during the grindingphase when dangling silicon bonds are exposed at the fracture plane.Accordingly, it is advantageous to perform the crushing in an inertambient, for example, of nitrogen or argon. The oxygen partial pressureshould be kept to less than 100 ppm (10⁻⁴) of the ambient pressure,which may be atmospheric or slightly over pressured. Further, themilling produces fine powder which presents an inhalation problem andsilicon powder may ignite and burn, that is, oxidize, in the presence ofoxygen. Both problems are greatly reduced by crushing within anenclosure filled with inert gas.

Although nitrogen may react with the silicon dangling bonds createdduring crushing, it is possible that the resultant very thin nitridelayer acts as a protective layer against subsequent oxidation of theunderlying silicon, which absent the protective layer would develop moredeeply into the ground silicon particle.

An example of a environmental processing chamber 100, schematicallyillustrated in FIG. 7 includes a vacuum-pumped or vented glove box 102.A nitrogen source 104 supplies nitrogen to the glove box 102 to achievethe desired low concentration of oxygen. The glove box 102 includes twoglove holes 104 with gloves sealing the interior from the exterior butallowing an operator to manually manipulate equipment and productswithin the nitrogen-filled glove box 102.

The crushing system 10 and the feed system 70 are located inside theglove box 102. The glove box 102 also includes a load lock 108 having anexterior vacuum door to ambient, an interior vacuum door to the interiorof the glove box 102 and an interior of sufficient size foraccommodating feedstock and ground product. In operation, feedstock isplaced from the exterior into the load lock 108 and its exterior door isclosed. After the load lock 108 has been backfilled with nitrogen to therequisite low oxygen level, the interior door is opened and the operatorworking through the glove holes 106 can transfer feedstock to the feedsystem. At the end of grinding a load of feedstock, the operator maytransfer the contents of the collector pan into a sealable bottle, whichis then transferred out of the glove box 102 through the load lock 108and a new load of feedstock may be loaded into the glove box 102.

The described processing system is effective at producing significantamounts of milled silicon particles since the crushing is substantiallya continuous process. However, it is understood that industrialproduction would be in large part automated and material wouldpreferably be nearly continuously loaded and unloaded from the inertprocessing ambient.

The crushed silicon may be sieved to obtain the appropriate size for jetmilling without damage to its silicon walls, for example, in the rangeof about 30 to 80 microns although somewhat smaller particles aredesired. Sieving also removes elongate particles having minor dimensionsless than the inter-roller gap 72 but a major dimension is larger thanthe inter-roller gap 72. Sieving has been performed with sieves ofdifferent screen mesh sizes having uniform hole sizes in the range of 30to 90 microns. The size of the mesh screen determines the maximum sizeof the particles passing the screen. A lower limit of the particle sizecan be achieved by a separate sieving step with a smaller mesh size andretaining the particles not passed through the sieve. Of course, sievingselects particles according to their minimum dimensions and irregularlyshaped particles having larger maximum dimensions than the screen meshmay nonetheless pass the mesh. The screen material should benon-metallic, for example, nylon. The sieving has demonstrated that theroller mill may produce particles considerably smaller than theinter-roller gap 72.

One mode of grinding produces both fine powder and larger particles,which may been partially crushed but not reduced to powder. Aftersieving, the larger particles may be milled multiple times to increasethe yield of fine powder.

Progressive grinding can be accomplished in at least two ways. In afirst approach, relatively large particles are ground with a relativelylarge inter-roller gap. The ground particles are collected, theinter-roller gap is reduced, and the particles are ground a second time.This process can be repeated more times. In a second approach, multipleroller mills 10 are stacked above each other with their respectiverollers 12, 14 approximately above each other and more importantly theirinter-roller gaps are approximately above each other so that the powderground in the uppermost roller mill is immediately ground again in thenext lower roller mill. The inter-roller gaps are selected to be largestfor the highest mill and to progressively decrease for the lower mills.It is even possible to use the same inter-roller gap among the stackedcrushers to increase the yield.

The silicon powder produced by the roller mill may be used as feedstockfor the silicon-lined jet mill or it may be used directly as thefeedstock for the silicon-lined plasma gun. The jet mill allows powdersize to be controllably reduced to a size of between 1 to 5 microns.

If the crushed silicon powder is to be further ground in a jet mill, thejet mill may additionally be placed within the glove box 102 and thecrushed particles transferred to the jet mill without being removed fromthe glove box 102.

The various aspects of the invention provide an economical anddependable process for producing silicon particles and powders of highpurity and controlled small size. If desired, the silicon powder isdoped to a controlled level by a simple and economical process. Theinvention provides economic feedstock for plasma spraying ofsemiconductor grade silicon.

1. A method of doping silicon powder, comprising: exposing siliconparticles to a liquid dopant capable of doping silicon to a givensemiconductivity type to create doped silicon particles; and pulverizingthe doped silicon particles to lesser size to create a first batch ofsilicon powder.
 2. The method of claim 1, further comprising plasmaspraying silicon using a silicon powder feed comprising the first batch.3. The method of claim 2, further comprising: pulverizing lesser-dopedsilicon particles of lesser doping than the doped silicon particles tocreate a second batch of silicon; mixing predetermined amounts of thefirst and second batches of silicon of which the silicon powder feed iscomprised.
 4. The method of claim 1, wherein the liquid dopant comprisesboric acid.
 5. The method of claim 1, wherein the liquid dopantcomprises phosphorus oxychloride.
 6. The method of claim 1, wherein theexposing step comprises spraying a predetermined amount the liquiddopant on the silicon particles.
 7. The method of claim 1, wherein thepulverizing comprises roller milling the silicon particles between twocounter-rotating rollers having cylindrical surface portions consistingessentially of elemental silicon.
 8. A roller mill adapted for millingsilicon, comprising: a pair of juxtaposed rotatable rollers havingsurface portions juxtaposing each other comprising elemental silicon, agap being formable between the rollers.
 9. The mill of claim 8, whereinthe gap has a controlled variable size.
 10. The mill of claim 8, furthercomprising end plates having surface portion comprising elementalsilicon biased against axial ends of the rollers.
 11. The mill of claim10, wherein corners of the rollers adjacent the end plates arechamfered.
 12. A roller mill system, comprising plural verticallystacked pairs of the rollers of claim
 8. 13. The mill of claim 8,further comprising a feed system reciprocating parallel to the axes ofand above the rollers for feeding feedstock to the rollers.
 14. The millof claim 13, wherein the feed system includes a linear funnel with alinear outlet.
 15. The mill of claim 14, wherein the linear outlet ispositioned above one of the rollers and laterally away from a gapbetween the rollers.
 16. The mill of claim 13, wherein the linear funnelis composed of elemental silicon.
 17. A method of pulverizing silicon,comprising: milling silicon in the roller mill of claim 8; and sievingthe product of the milling step to a predetermined size range.
 18. Themethod of claim 17, wherein the size range is 20 to 80 microns.
 19. Themethod of claim 17, wherein the predetermined size range extendsentirely over sizes less than the gap between the rollers.
 20. Themethod of claim 17, further includes jet milling the sieved product.